In the solid state 9-tert-butylanthracene exists in a structure where the anthracene ring is nonplanar. The dihedral angle between the two lateral benzenic rings is about 18°. In addition the tertiary butyl group is displaced in a direction opposite to the ring bending. Ab initio molecular orbital calculations (3-21G) agree that the isolated (gas phase) molecule exists in a nonplanar geometry and indicate that there are two mechanisms for tert-butyl group rotation. A high-energy (35.1 kJ/mol) rotation requires a further bending of the tert-butyl group. The low-energy process (4.7 kJ/mol) involves a concerted tert-butyl rotation and butterfly-like inversion (flapping) of the anthracene fused ring. The motion of the tert-butyl group in solid 9-tert-butylanthracene was monitored by variable temperature 13 C CP/MAS NMR spectroscopy and appears to follow the highenergy mechanism for tert-butyl rotation with an activation energy of 63.1 ( 2.7 kJ/mol. The nearly 30 kJ/mol increase over the calculated value is the result of additional intermolecular interactions in the solid state. In solution (5:2 CS 2 /acetone-d 6 ) it was not possible to stop the tert-butyl group rotation even at the lowest achievable temperature (ca. 150 K). This is consistent with the lowenergy mechanism, where the ring inversion is not prevented by crystal packing forces.